DVB-T (Terrestrial Digital Video Broadcasting) is the digital terrestrial television standard adopted by Europe and many other countries. A very important operation in a mobile DVB-T digital terrestrial television demodulator is the estimation of the time-varying channel. If this can be done accurately, then other functions like equalization and inter-carrier-interference cancellation are made simpler.
DVB is the European consortium standard for the broadcast transmission of digital terrestrial television. DVB systems transmit a compressed digital audio/video stream, using multi-carrier modulation, such as orthogonal frequency division multiplexing (OFDM).
DVB-T broadcasters transmit data with a compressed digital audio-video stream using a process based on a Moving Picture Expert Group (MPEG)-2 standard. These transmissions can include all kinds of digital broadcasting, including high definition television (HDTV). MPEG-2 signals represent an improvement over the older analog signals, which require separate streams of transmission.
By way of background, in multi-carrier systems, such as OFDM systems, serially-inputted symbol streams are divided into unit blocks. The symbol streams of each unit block are converted into N number of parallel symbols. After the conversion, these symbols, which include data, are multiplexed and added by using a plurality of subcarriers having different frequencies, respectively, according to an Inverse Fast Fourier Transform (IFFT) technique, and are transmitted via the channel in time domain.
In addition to data, these OFDM symbols also include scattered pilot carriers (SPC), continuous pilot carriers (CPC), and reserve tone pilot carriers. These pilot carriers (signals) are used for frame synchronization, frequency synchronization, time synchronization, channel estimation, transmission mode identification, and/or phase noise tracing. The data and the pilot carriers constitute the useful part of the OFDM symbol.
Once the OFDM symbols are captured on a receiver side of the OFDM system, they must be demodulated. OFDM demodulation procedures include, for example, a Fast Fourier Transform (FFT) step, an equalizing and de-interleaving step, and a synchronizing step, among others.
Synchronization of OFDM receivers is performed to locate the useful part of each symbol to which the FFT is to be applied. This synchronization, generally performed in the time domain, can be characterized as coarse synchronization (e.g., initially performed during an acquisition period) and fine synchronization. Fine synchronization improves upon the results achieved during coarse synchronization enough to provide reliable demodulation. Time domain correlation is used to achieve initial symbol timing synchronization.
DVB-T2 is the second generation DVB Digital Terrestrial Television standard that has recently undergone standardization. This is OFDM-based and has FFT sizes up to and including 32K. One problem in DVB-T2 demodulation is equalization, and in particular, accurately estimating the channel frequency response. Once the channel frequency response is known, the equalization may be carried out using known MMSE algorithms. Furthermore, from the channel frequency response, the channel impulse response can be calculated and this may be used for symbol timing recovery. The conventional method given in DVB-T2 Implementation Guidelines is temporal interpolation of scattered pilots. In fact, the DVB-T2 standard has specifically introduced symbols like the preamble symbol P2 and the frame closing symbol with additional pilots to facilitate this temporal interpolation at the beginning and end of frames.
Temporal interpolation does not by itself give a good channel estimate in noisy conditions. It is necessary to create a low-pass temporal filter to get a good enough channel estimate. Temporal interpolation and temporal filtering are simple to implement but are very expensive in memory utilization. This is because these operations have to be implemented in the time dimension. The symbols may be as big as 32K in the frequency dimension and to implement a temporal filter several symbols have to be stored. Channel estimates are known at scattered pilot locations. These estimates have to be interpolated (or rather low-pass filtered) to get the channel estimates at data carrier locations. So to get the channel response for symbol n it is necessary to wait for symbols (n+1), (n+2), (n+3) etc. to arrive. Since the symbols are long, this implies a significant storage requirement in the demodulator. This translates to a significant die size cost and it is known that OFDM based digital TV demodulators are dominated by memory.